This invention relates to a wound multiple-capacitor filter for suppressing radio frequency (R. F.) signals on power lines.
Such filters are commonly used in association with electrically powered appliances which tend to generate noise signals that interfere with nearby radio and television reception. These filters normally include one capacitor connected across one phase of the power-lines to filter the "symmetrical" component of the noise signal, and are capacitor connected between each power-line to ground to filter the "common-mode" components of the noise signal. For a single phase line the filter usually has three capacitors in a delta (.DELTA.) circuit configuration.
The incorporation of such filters to an electrical device or appliance causes an increased level of AC leakage current (of power-line frequency) to ground. Various standards limit the total AC leakage current contributed by both the device and the filter to no more than from 0.75 to 5 milliamperes (ma). Higher currents may prove lethal to personnel who may be touching the "grounded" housing of the device when the wire to earth ground is inadvertently opened. The AC leakage current attributable to the filter is about directly proportional to the capacity of one of the aforementioned line-to-ground capacitors, thus limiting the size of these capacitors that may be used. On the other hand the diverting of the common-mode noise signals from the power lines is enhanced by increasing the size of the line-to-ground capacitors.
The filter designer is faced with the additional consideration that a failure by shorting of a line-to-ground capacitor would pose a serious shock hazard. These capacitors which may operate under the full line-to-line voltage must therefore be of high quality and have a long expected life under these operating conditions. For example, the characteristic corona start voltage of the line-to-ground capacitors should be substantially greater than the line-to-line voltage to avoid early capacitor failure. These considerations lead to high costs in the production of line-to-ground filter capacitors.
It is therefore an object of this invention to provide a reliable, low cost interference filter having a high noise-rejection to AC-leakage-current ratio.